1. Field of the Invention
The present invention generally relates to a method and apparatus for monitoring transient physical phenomenon such as seismic activity and to monitor the passage of surface and subsurface "anomalies" such as surface vessels or submarines. More specifically, the present invention relates to monitoring transient physical phenomenon or the passage of anomalies by monitoring changes in an alternating component of a generally vertical current which emanates from the earth.
2. Description of the Prior Art
As detailed in U.S. Pat. No. 4,507,611, the contents of which are hereby incorporated by reference, many techniques are known in the prior art for performing geographical prospecting. For example, Ruehle, et al., in U.S. Pat. No. 3,363,457, teaches that the measurement of radiant energy from subsurface formations enables geophysical prospecting. Weber, in U.S. Pat. No. 4,044,299 teaches a prospecting technique which includes the use of an inductive exciter which induces alternating current energy into the area and structure of the earth which is to be observed. Measurement of the induced current energy enables an artisan to determine the underground environment of the area.
A method and apparatus for measuring subsurface electrical impedance utilizing first and second successively transmitted signals at different frequencies is taught by Madden, et al. in U.S. Pat. No. 3,525,037.
In U.S. Pat. No. 3,942,101 Sayer teaches a geophysical prospecting device which utilizes a distortion of the atmospheric electrostatic potential gradient which is suggested to be a result of the Nernst effect. Sayer teaches that the distortion provides a means for locating subterranean sources of geothermal energy.
The earth's electromagnetic field also has naturally occurring alterations of the type known as "magnetic noise". Slichter, in U.S. Pat. No. 3,136,943, discloses that such noise is the product primarily of lightening and other electrical discharges and phenomenon. Geothermal prospecting can be performed by detecting variations in the naturally occurring electromagnetic radiations from thunderstorms or other phenomenon. The detection and measurement of short-term variations in the earth's magnetic field for geothermal prospecting is described in U.S. Pat. No. 3,126,510 to McLaughiin.
Prospecting can also be performed by comparing simultaneous variations of an underground electric field and a magnetic field which results from the circulation of telluric currents. This comparison of electric and magnetic fields requires the use of electrodes to measure the internal telluric currents in the magnetic field according to Cagniard; see U.S. Pat. No. 2,677,801.
The above mentioned generally vertical current having an alternating component is distinct from the above-mentioned telluric currents for many reasons. Firstly, telluric currents are usually direct currents. Secondly, telluric currents occur only within the earth whereas the generally vertical current having an alternating component emanates from the earth's surface over land and water. Thirdly, telluric currents exhibit local discontinuities and are not based on ionic impingement of solar winds. In contrast, the generally vertical current having an alternating component has generally predictable time variations due to the constant directivity (generally vertical), depth, and diurnal character.
A 1982 publication by the Soviet Academy of Sciences entitled Electro-Magnetic Precursor to Earthquakes includes a passage in reference to measurements made using a pair of electrodes, one positioned at the bottom of a five-hundred meter deep shaft and the other at the top of that shaft. During a period of seismic activity, electrical currents having high frequency components were measured between the electrodes.
Machts et al., in U.S. Pat. No. 2,124,825, describe an apparatus for investigating the electric and magnetic field conditions in an area being surveyed in order to locate irregularities indicative of earth strata, rock fractures, oil and water-bearing earth formations, etc.
Stanton, in U.S. Pat. No. 2,659,863, describes a method and apparatus for determining the presence of oil, mineral, and other subterranean deposits by measuring variations in a vertical potential gradient in the atmosphere near the surface of the earth.
Morrison, U.S. Pat. No. 2,784,370, describes a prospecting device for locating subterranean anomalies which utilize measurements of electrical potential at or near the surface of the earth which results from "terrestitial electricity".
Barringer, in U.S. Pat. No. 3,763,419, describes a method and apparatus for geophysical exploration which utilizes very low frequency fields produced by distant transmitters as a source of a primary field. A vertical component of that field is used as a stable reference against which variations caused by discontunities in the earth conductivity can be measured.
Miller, et al., in U.S. Pat. No. 4,041,372, describes a system which utilizes a current source to provide an input signal having predetermined frequency components, amplitude relationships, and duration. A plurality of spaced-apart detectors are used to make differential electrical measurements which permit cross correlation with the input signal.
In summary, the prior art teaches geophysical prospecting which utilizes variations in naturally occurring electrostatic potential gradient, alternations in the earth's electromagnetic field, short-term variations in the earth's magnetic field and simultaneous variations of the underground electric field and magnetic field and other devices which require the use of induced primary fields. These prior art techniques are in contrast to the present invention which utilizes a generally vertical alternating current emanating from the earth, the existence of which is confirmed by the Soviet Academy of Sciences report.